In the prior art a slip mechanism gear is usual placed on a pinion and is coupled thereto by being urged strongly toward the pinion by a spring with a cork facing between the gear and the pinion so as to provide a frictional coupling between them. This method however has a disadvantage from the point of view of economy since it is necessary to provide a cork friction surface between the gear and the pinion.
As another prior art method it is known to form a metal gear and to integrate the metal gear with the pinion by molding with a plastic material. In this method however the slip torque is obtained by a holding force produced between the metal and plastic at the time of molding. Therefore the slip torque is subject to great variations with slight changes of the plastic.
In prior art slip mechanisms in which one or both of the pinion and gear are made of metal it has been necessary to resort to mechanical coupling means and also lubrication is required for the purpose of stabilizing the slip torque.